The invention concerns a method for dry pelletizing of powdered carbon black with control of the pellet quality while increasing the output of a particular pelletizing machine. This is accomplished by pre-compacting the initial carbon black to be pelletized and pelletizing by means of rollers suited to implement one version of this method.
The carbon black obtained in the various production facilities is in the form of a finely distributed powdery material, giving rise to much dust. For the purpose of simpler and more economical handling, much of the produced carbon black is converted into free-flowing pellets.
On one hand, it is known how to wet-granulate carbon black. A specific amount of liquid is added to carbon black in a suitable apparatus, and granulation takes place by a constraining action on such a mixture. On the other hand, dry granulation is known for only very special types of carbon black compounds, whereby the carbon black is converted into granules by rolling motions of variable duration.
Even though almost the entire range of carbon black materials can be made granular by the wet granulation method, and even though the pouring properties, strengths, and flow properties of the pellets so made can be varied over wide ranges, there are significant factors which make wet granulation undesirable.
Mixing the carbon black with a liquid pelletizing medium requires an additional application of energy so that pelletizing can occur. Furthermore, if pelletizing is to be especially soft, which is frequently desired in specific fields of application, wet granulation will offer unsatisfactory results. The addition of binders in the wet granulation process often is a source of interference in subsequent processing. Lastly, a large amount of energy is required to re-evaporate the previously added pelletizing medium, which represents a comparatively uneconomical step.
Only a restricted number of carbon black compounds may be subjected to dry pelletizing methods employed heretofore, and most carbon black compounds could not be pelletized by it.
In the prior art devices, specific pellet parameters such as bulk density, hardness of the individual pellet, pellet size and the pelletizing capacity of the apparatus were largely predetermined relatively to the desired end product. The parameters could be varied only within very narrow limits. Controlled adjustment of the above cited parameters was substantially impossible.
The known procedures were too inflexible to allow an operator to control the pellet quality. They predetermined the pelletizing capacity, and this capacitY could not be raised.
The German Auslegeschrift No. 10 51 815 discloses a process for the dry granulation of powders in the absence of binders for the purpose of controlling the quality of the granulate (desired grain size, granulate hardness). This process compacts the pigment into a flaky intermediate product by compression. The intermediate product subsequently must be comminuted and sifted before the conventional pelletizing step. Accordingly, the procedure consists of: mechanical compression, comminution of the compressed material, sifting and granulation proper. This granulation proper therefore takes place on highly compacted cores obtained from the sifting step. Consequently, the core and the shell of the end pelletized product are compacted non-uniformly, that is, the cross-sectional compression density is nonhomogeneous.
Raw materials, such as carbon black which displays a characteristic particle structure, cannot be pelletized by the above-mentioned procedure unless the characteristic structure is altered. This is because the strong mechanical compression during pre-compaction causes degradation of the characteristic structure with undesirable consequences on properties which are important for further processing and applications.
U.S. Pat. No. 2,883,274 describes an apparatus and a procedure to pelletize and compact carbon black. The apparatus consists of a rotatable cylindrical drum mounted horizontally and a vertical weir mounted to the inside wall of the drum which divides the drum into a pelletizing chamber and a compaction chamber.
In order to move the agglomerated pellets formed in the pelletizing chamber over the weir and into the compaction chamber, use is made of shovel-like means mounted to the weir at the level of the overflow apertures. This shovel-like means communicates with the apertures, scoops up small quantities of pellets from the surface of the pelletizing bed during drum rotation. These scooped-up pellets are stacked until a specific overflow height is attained, at which time the pellets are discharged through the overflow apertures. The pellets fall into the compaction chamber wherein they are further compacted by rotation. When compaction is complete, the pellets are removed from the drum.
Accordingly, the invention described in the above-mentioned patent comprises a pelletizing step followed by a compaction step. This method produces pellets which are highly compacted by mechanical means.